Increasing demands in fuel efficiency has made hybrid systems more attractive in the automotive industry. An electric motor may be an important part of a hybrid system, and may be configured as an alternating current (AC) induction motor including a stator and a rotor. The stator is the outermost component of the motor and is composed of steel laminations including teeth shaped to form poles with copper wire coils wound around the poles to form windings. The primary windings are connected to a voltage source to produce a rotating magnetic field. The rotor is the innermost sub-assembly of the motor and may be composed of a stack of steel laminations including teeth shaped to form poles, which are separated by conductor bars electrically connected to end rings located at opposing ends of the stack. The interaction of currents flowing in the conductor bars of the rotor winding and the stator's rotating magnetic field generates torque.
One method of constructing an aluminum rotor is to cast an aluminum alloy into the slots of the lamination stack while simultaneously casting aluminum end rings to create an electrical circuit, using high pressure die cast (HPDC) methods. This arrangement of cast rotor bars and cast end rings resembles a squirrel cage, leading to the name squirrel-cage induction motor. A major barrier to widespread implementation of HPDC aluminum induction rotor components is the poor integrity of the aluminum cage, also referred to as a rotor frame, formed by the HPDC process. Casting defects observed in the HPDC aluminum rotors such as entrained air bubbles, hot-tearing cracks, knit lines, shrinkage, porosity, and oxide inclusions, for example, can significantly reduce electrical and mechanical performance of the rotor.
In addition, the aluminum alloys used to cast rotor squirrel cages are usually high purity aluminum alloy such as AL99.7 (99.7% pure aluminum having a conductivity of 62% IACS measured using the International Annealed Copper Standard (IACS)), or electric grade wrought alloys which are all difficult to cast because of the low fluidity and high shrinkage rate of these alloys. These characteristics of the higher purity aluminum alloys increase porosity and the tendency for hot tearing or other discontinuities, particularly at the locations where each of the cast conductor bars intersect a casting forming one of the end rings, which can lead to fracture between the conductor bars and the end rings.
Furthermore, the high velocity flow rates of the molten aluminum used to fill the thin long conductor bars (squirrel slots) in the laminate steel stack, which may be up to 60 m/s, produce turbulent fill which tends to entrain mold cavity gas and generate abundant aluminum oxides during the casting process. Turbulent metal fill during HPDC of the end rings and the conductor bars decreases rotor quality and durability and also significantly reduces the thermal and electric conductivity of the rotor frame, particularly in the conductor bars. In practice, the electric conductivity of an aluminum rotor frame (cast conductor bars integrated with cast end rings) formed by current HPDC casting methods may only be about 40 to 45% IACS. The casting defects present in the HPDC aluminum conductor bars may reduce conductivity and significantly affect motor performance.